The present invention relates to an apparatus and method for producing nitrogen in which air is rectified within a single column to produce a nitrogen rich tower overhead. More particularly, the present invention relates to such a method and apparatus in which reflux for the column is produced in a head condenser utilizing a coolant stream composed of a nitrogen containing liquid having a nitrogen content greater than that of oxygen-rich liquid produced as column bottoms within the column. Still even more particularly, the present invention relates to such an apparatus and method in which the coolant is recompressed and then liquefied within a reboiler contained within the bottom of the column prior to its reintroduction into the column.
Nitrogen is produced in a single column plant known as a nitrogen generator. In such a plant, air is compressed, purified, and cooled to a temperature suitable for its rectification, normally, at or near dewpoint temperature of the air. The air is then introduced into the column and rectified to produce a nitrogen rich tower overhead and an oxygen rich column bottoms. The column can be designed with a sufficient number of stages to produce high purity and even ultra-high purity nitrogen as a product.
An example of such a single column nitrogen generator is shown in U.S. Pat. No. 5,582,034. In this patent, coolant for the column head condenser consists of two streams, one of which is composed of the oxygen-rich column bottoms and the other of which is composed of a nitrogen containing liquid having a higher nitrogen content than the oxygen-rich column bottoms. The nitrogen containing liquid after having been vaporized is recompressed and then reintroduced into the bottom of the column. The coolant stream composed of the oxygen-rich liquid is partially warmed and then expanded within a turbo expander to produce refrigeration for the plant.
The re-introduction of the coolant, composed of the nitrogen containing liquid, increases recovery of the nitrogen product. However, since the nitrogen containing liquid has a higher nitrogen content than the oxygen-rich liquid, it has a higher dewpoint pressure for the same temperature of the oxygen-rich liquid. In other nitrogen generators it is the oxygen-rich liquid that solely serves as the coolant and is reintroduced into the column to increase recovery. However, when the nitrogen containing liquid is used for this purpose, since it has the higher dewpoint pressure, the required compression is lower for the same amount of production, and therefore there are efficiencies of operation that are realized in this patent.
As will be discussed, however, the apparatus and process of U.S. Pat. No. 5,582,034 can be further modified to produce similar nitrogen recoveries.